Synergistic Effects in N,O‐Comodified Carbon Nanotubes Boost Highly Selective Electrochemical Oxygen Reduction to H2O2

Abstract Electrochemical 2‐electron oxygen reduction reaction (ORR) is a promising route for renewable and on‐site H2O2 production. Oxygen‐rich carbon nanotubes have been demonstrated their high selectivity (≈80%), yet tailoring the composition and structure of carbon nanotubes to further enhance the selectivity and widen working voltage range remains a challenge. Herein, combining formamide condensation coating and mild temperature calcination, a nitrogen and oxygen comodified carbon nanotubes (N,O‐CNTs) electrocatalyst is synthesized, which shows excellent selective (>95%) H2O2 selectivity in a wide voltage range (from 0 to 0.65 V versus reversible hydrogen electrode). It is significantly superior to the corresponding selectivity values of CNTs (≈50% in 0–0.65 V vs RHE) and O‐CNTs (≈80% in 0.3–0.65 V vs RHE). Density functional theory calculations revealed that the C neighbouring to N is the active site. Introducing O‐related species can strengthen the adsorption of intermediates *OOH, while N‐doping can weaken the adsorption of in situ generated *O and optimize the *OOH adsorption energy, thus improving the 2‐electron pathway. With optimized N,O‐CNTs catalysts, a Janus electrode is designed by adjusting the asymmetric wettability to achieve H2O2 productivity of 264.8 mol kgcat –1 h–1.

recorded on an X-ray powder diffractometer (MeasSrv F9XDZ42) with Cu Kα (λ = 0.154 nm) radiation at a scan rate of 10 ºC min-1 in the 2θ range from 5 to 80°. X-ray photoelectron spectroscopy (XPS) was recorded on a Thermo Electron ESCALAB250 XPS Spectrometer (X-ray Source: Al). The C1s line at 284.8 eV was utilized to reference the binding energies in the acquired spectra. The concentration of Ti 4+ was measured by UV-vis spectrometer (Shimadzu 2600) at 408 nm.

Electrochemical measurements
Electrochemical data were collected using WaveDrive 10 electrochemical workstation (Pine Research Instrumentation, USA). A three-electrode system was used for electrochemical measurement. The saturated calomel electrode (SCE) was used as a reference electrode, platinum plate electrode was used as the opposite electrode, disk area was 0.247 cm -2 , and the working electrode was a platinum ring (ring area was 0.168 cm -2 ). ORR occurs on the disk electrode, and H 2 O 2 generated in situ is continuously oxidized on the ring electrode at 1600 rpm. All potentials measured at SCE were converted to reversible hydrogen electrode (RHE) potentials. To prepare the working electrode, a 4.1 mg sample and 20 µL Nafion solution (D520, 5%) were added into 0.98 mL isopropyl alcohol under ultrasonically mixing to get the homogeneous ink. RRDE measurement was then carried out by uniformly dropping 6 µL ink onto the disk electrode using a rotary coating device. LSV test was performed in O 2 -saturated 0.1 M KOH solution with a scan rate of 5 mV s -1 at 1600 rpm. The applied potential on Pt ring during the LSV test was set as 1.5 V versus RHE to record the ring current. [ Where I ring is the ring current, I disk is the disk current and N is the collection efficiency of the RRDE.
The electrochemically active surface area was measured by double layer capacitance method.
CV scans were conducted at the potential window from 0.96 to 1.04 V versus RHE reference electrode with scan rates of 5, 10, 15, 20, and 25 mV s −1 . By plotting the (Ja − Jc)/2 at 1.0 V versus RHE against the scan rate (Ja is the anodic current density and Jc is the cathodic current density), the slope value was calculated to be the double layer capacitance (C dl ).

Wettability regulation of membrane electrode
The preparation process of asymmetric wettability Janus electrode is shown in Figure S16.
Hydrophobic carbon paper (purchased from Toray) was cleaned with hydrochloric acid,

H 2 O 2 concentration measurement
The electrochemical measurement of H 2 O 2 production was performed using a custom H-cell.
As shown in Figure S18, the Janus electrode was used as the working electrode to electrochemical catalysis produce H 2 O 2 in a two-compartment cell. The counter electrode is a platinum sheet electrode, the reference electrode is a saturated calomel electrode, the electrolyte is 1.0 M KOH, and Nafion 117 membrane is a separator.
The H 2 O 2 concentration was measured by a traditional titration method based on the mechanism that a colorless solution of Ti (SO 4 ) 2 would be oxidized by H 2 O 2 to H 2 TiO 4 with yellow color in neutral and acidic environments.

T T
Thus, the concentration of H 2 O 2 after the reaction can be measured by UV-Vis spectroscopy.
The wavelength used for the measurement of H 2 TiO 4 was 408 nm. In order to build an acidic environment, 10 mM Ti (SO 4 ) 2 and 2.0 M H 2 SO 4 were mixed evenly as a chromogenic agent.
H 2 O 2 with a known concentration of 24, 48, 96, 120 ppm was mixed with the chromogenic agent at a volume ratio of 1:1. The absorbance of its UV-Vis at 408 nm was measured. After linear fitting, the standard curve was drawn ( Figure S19). During the H-cell electrolysis of N, O-CNTs, the electrolyte at 0, 5, 10, 15, and 20 min was taken for color development, and its absorbance was measured and incorporated into the standard curve to obtain the actual H 2 O 2 content.

Computational details
All DFT calculations were constructed and implemented in the Vienna ab initio simulation package (VASP). [2] Using the electron exchange and correlation energy was treated within the generalized gradient approximation in the Perdew−Burke−Ernzerhof functional (GGA-PBE) (GGA-PBE), [3] the calculations were done with a plane-wave basis set defined by a kinetic energy cutoff of 450 eV. The K-point sampling was obtained from the Monkhorst−Pack scheme with a (3 × 3 × 1) mesh for optimization. The geometry optimizations and energy calculations are converged when the electronic self-consistent iteration and force reach 10 -5 eV and 0.02 eV Å −1 , respectively.

Models
The diameter of nanotube is 5.

The Gibbs Free Energy Variation
The change in Gibbs free energy (ΔG) of each adsorbed intermediate is calculated based on the computational hydrogen electrode method developed by Nørskov et al. [4] At standard condition (T = 298.15 K, pH = 0, and U = 0 V (vs. SHE)), the free energy G is defined as the following equation: Where △E is the energy change obtained from DFT calculation, △E ZPE is the difference between the adsorbed state and gas, which was calculated by summing vibrational frequency for all models based on the equation: E ZPE = 1/2∑hv i (T is the temperature (298.15 K) in the above reaction system, and △S represents the difference on the entropies between the adsorbed state and gas phase. The entropies of free molecules were obtained from NIST database (https://janaf.nist.gov/).

Adsorption energy
Including *OOH, *O and *OH, it was calculated relative to H 2 O and H 2 under conditions of T = 298.15 K, pH = 0, and U = 0 V (vs. SHE) according to following equations: Where * represents the adsorption sites associated with nanotube. The above △Gads are defined as the reaction free energies of the following reactions.